The present invention relates to a peak/bottom detection circuit for detecting the peak level or bottom level of a laser output, and more specifically, to a peak/bottom detection circuit used for controlling the laser output of an optical head for rewritable optical disks (CD-R, CD-RW, DVD-RAM).
FIG. 1 is a block diagram showing the structure of a conventional peak detection circuit, and FIG. 2 is a diagram showing the waveform of its operation.
As shown in FIG. 1, an input signal SA is supplied to an input terminal 50, and a peak hold signal SB is output from an output terminal 54.
An operation amplifier 51 compares the input signal SA with the peak hold signal SB, and while the input signal SA is being higher than the peak hold signal SB (SA&gt;SB), the output from the operation amplifier 51 is set at "H" level, and the diode 52 is turned on. The diode 52 supplies a charge current ICC to a hold capacitor 53, so as to charge the hold capacitor 53. The level of the peak hold signal SB increases as the hold capacitor 53 is charged as shown in FIG. 2.
Further, when the level of the input signal SA is set to that of the peak hold signal SB or less (SA.ltoreq.SB), the operation amplifier 51 sets its output to "L" level, and the diode 52 is turned off. Thus, the peak hold signal SB is set in a hold state as shown in FIG. 2.
As the above-described operation is repeated sometimes, the peak hold signal SB is held at substantially the peak level of the input signal SA.
However, in the peak detection circuit shown in FIG. 1, the charge current ICC is not controlled, and therefore it is difficult to finish charging the hold capacitor 53 quickly. Therefore, as can be seen in FIG. 2, the peak hold signal SB, in some cases, exceeds very much the peak level of the input signal SA. This results in a detection error ERR, which greatly decreases the accuracy of the peak detection.
Further, in some other cases, the peak detection is stopped in the middle of its operation, and set in a hold state. In the circuit shown in FIG. 1, it is necessary to switch the input signal SA to the level of the peak hold signal SB or less, in order to execute such a control as just described. This control causes a complexity to the laser output control system or peak detection circuit, which leads to an increase in the product cost.
Further, when the input signal SA is switched, a switching noise is generated. If the peak detection circuit detects the switching noise, a highly accurate peak detection will become further difficult.